This invention relates to a signal level detector employing diode circuits which are arranged to extend the continuous input signal range over which the detector can operate without user intervention to modify the detector circuits.
Signal level detectors often employ a diode envelope detector type of circuit. Such diode circuits are characterized by a limited dynamic range of input signals in which they can work, and they are highly sensitive to temperature variations. Many detector applications require a high degree of operating precision over a large signal range and in a wide temperature range necessitating complex and expensive measures to cope with the problems of temperature sensitivity and limited dynamic range of operation.
Diodes employed in such diode envelope detectors are often the so-called Schottky type of diode. These detector circuits are often divided according to two different types of operation. One is a low input signal, or square-law, detector in which the output voltage is a function of the square of the input voltage, or a function of the input power. Another type is the large signal, or linear-law, detector in which the output voltage is a function of the input voltage. The diode sensitivity to temperature variations is most severly evidenced in the square-law region of operation. It is known that diode detector circuits can be modified in order to alter either or both of the temperature sensitivity and the range of operation. In some cases, the circuit modifications must themselves be changed when the range of operation of the detector is to be altered. Consequently, envelope detectors which must operate under severe temperature conditions and over a large dynamic range of input signals, become quite complex and expensive.
An F. N. Eastland U.S. Pat. No. 4,000,472 shows an envelope detector with temperature compensation and having a bias voltage applied to the detector input to shift detector operation out of the nonlinear square-law region of operation. The temperature compensation is achieved by having a signal path extending through the envelope detector to one input of a voltage difference amplifier, and having a similar reference path including similar diodes extending to another input of the same difference amplifier. The difference voltage between the two paths is relatively less affected by temperature variations. A paper by R. J. Turner showing a similar temperature compensating technique and entitled "Schottky Diode Pair Makes an RF Detector Stable" is found in the May 2, 1974, issue of Electronics at pages 94 and 95.
One form of a current bias arrangement for an envelope detector diode is shown at pages 498-500 of Communication Circuits: Analysis and Design, by K. K. Clarke et al., Addison Wesley Publishing Company, Reading, Mass., 1971. A W. W. Snell, Jr. U.S. Pat. No. 3,241,079 shows a square-law detector having associated diodes for extending the square-law range of operation of the detector.